(a) The Field of the Invention
This invention relates to the art of overturning the sheets fed successively and printed on offset printing machines, namely for carrying out so called "white and perfecting" printing for obtaining, in a multi-colour printing machine including a plurality of printing units arranged and operating in series (independently of the division of these units into either individual or multiple structural groups) the impression or printing of the sheet on both faces in one or more colours, the sum of the colours printable on the two faces obviously not exceeding the number of the printing units of the machine.
(B) The Prior Art
Offset printing machines are well known and no further comments thereon are necessary, except the following, for the purpose of a better understanding of the present invention. In these machines, every printing or operating unit includes a series of counter-rotating cylinders, such as a plate cylinder effecting impressions on the surface of a rubber cylinder, which in turn conveys the impression onto a sheet wound on a printing cylinder. In said multi-colour sheet printing machines, the subsequent printing units, each provided with said series of cylinders, are interconnected by cylinders or chains or equivalent rotating or circuiting "transfer" means (which will therefore be termed, in the following specification and claims "transfer means" even if they will be described as consisting of transfer cylinders). These means transfer, under conditions of synchronism and phased timing, the successive sheets from a printing unit to a downstream printing unit.
Said units obviously also comprise structural components, and means for driving, wetting, inking, feeding, discharge and so on, not relevant to the invention.
The passage of the sheets from the printing cylinders to the transfer means and conversely, all counter-rotating at identical tangential speed, is performed by controlled grippers associated with check-dowels, and actuated so that the grippers of the component releasing the sheet (printing or transfer) bring the front edge thereof to a position tangentially adjacent to the component receiving it (transfer and printing) and release such edge whilst the grippers of the receiving component then engage it. All the sheets run through the entire machine, effecting curved movements in an alternately opposite directions. In these multi-colour printing machines, effecting multi-colour printing on the same face of the sheets, this impression or printing is received on the "convex" face of the sheet during its passage around the former and all subsequent printing cylinders.
In order to provide printing on both faces of the sheets, there are used printing machines, provided with a so called "white perfecting device", imposing on the sheet (and obviously on all subsequently fed sheets) a more intricate path. This white perfecting device is such as to cause the "concave face" of the sheet, during its passage around the upstream printing cylinders, to become the "convex face" of sheet. Generally speaking, the said white perfecting devices are internal to a transfer cylinder and involve considerable difficulties in construction, mounting, adjustment and service of the machine, whilst imposing on the sheets a most intricate path of travel.